Concerted action of two novel tRNA mtDNA point mutations in chronic progressive external ophthalmoplegia. Issue 2 (29th April 2008)

• Record Type: Journal Article
• Authors: Kornblum, Cornelia; Zsurka, Gábor; Wiesner, Rudolf J.; Schröder, Rolf; Kunz, Wolfram S.
• Is Part Of: Bioscience reports. Volume 28:Issue 2(2008)
• Page Start: 89
• Page End: 96
• Publication Date: 2008-04-29
• Subjects: chronic progressive external ophthalmoplegia (CPEO) -- mitochondrial DNA (mtDNA) -- mitochondrial DNA disease -- mitochondrial tRNA -- oxidative phosphorylation; Molecular biology -- Periodicals; Cytology -- Periodicals; 572.8
• DOI: 10.1042/BSR20080004
• Languages: English
• ISSNs: 0144-8463

Abstract

Abstract : CPEO (chronic progressive external ophthalmoplegia) is a common mitochondrial disease phenotype in adults which is due to mtDNA (mitochondrial DNA) point mutations in a subset of patients. Attributing pathogenicity to novel tRNA mtDNA mutations still poses a challenge, particularly when several mtDNA sequence variants are present. In the present study we report a CPEO patient for whom sequencing of the mitochondrial genome revealed three novel tRNA mtDNA mutations: G5835A, del4315A, T1658C in tRNA Tyr, tRNA Ile and tRNA Val genes. In skeletal muscle, the tRNA Val and tRNA Ile mutations were homoplasmic, whereas the tRNA Tyr mutation was heteroplasmic. To address the pathogenic relevance, we performed two types of functional tests: (i) single skeletal muscle fibre analysis comparing G5835A mutation loads and biochemical phenotypes of corresponding fibres, and (ii) Northern-blot analyses of mitochondrial tRNA Tyr, tRNA Ile and tRNA Val . We demonstrated that both the G5835A tRNA Tyr and del4315A tRNA Ile mutation have serious functional consequences. Single-fibre analyses displayed a high threshold of the tRNA Tyr mutation load for biochemical phenotypic expression at the single-cell level, indicating a rather mild pathogenic effect. In contrast, skeletal muscle tissue showed a severe decrease in respiratory-chain activities, a reduced overall COX (cytochrome c oxidase) staining intensity and abundant COX-negative fibres. Northern-blot analyses showed a dramatic reduction of tRNA Tyr and tRNA Ile levels in muscle, with impaired charging of tRNA Ile, whereas tRNA Val levels were only slightly decreased, with amino-acylation unaffected. Our findings suggest that the heteroplasmic tRNA Tyr and homoplasmic tRNA Ile mutation act together, resulting in a concerted effect on the biochemical and histological phenotype. Thus homoplasmic mutations may influence the functional consequences of pathogenic heteroplasmic mtDNA mutations.